This invention relates generally to electromagnetic wave resonators, and more particularly to electromagnetic wave resonators adapted for use in laser gyroscope apparatus.
As described in U.S. Pat. No. 3,741,657 entitled "Laser Gyroscope," Keimpe Andringa inventor, issued June 26, 1973 and assigned to the same assignee as the present invention, a laser ring resonator supports four optical waves, each one of such waves having a different frequency, one pair thereof traveling in a clockwise direction and the other pair thereof traveling in a counterclockwise direction. The optical pathlengths of the waves are such that the pair of frequencies of the waves traveling in one direction, say the counterclockwise direction, is positioned between the frequencies of the waves traveling in the opposite, or clockwise, direction.
Having established this frequency relationship, movement of the laser resonator, for example by rotation of the system about an axis perpendicular to the optical path, produces frequency shifts of the pair of waves propagating in one direction through the laser which are opposite to frequency shifts of the waves moving in the opposite direction through the laser. This, in turn, produces changes in the frequency separation between the lower frequencies of each of said pairs. The difference between such changes is, substantially, a linear function of the rate of said rotation and the relative sense of such difference is indicative of the direction of said rotation.
As described in the referenced U.S. Pat. No. 3,741,657, such frequency separation results from disposing in the path of the waves a polarization dispersive structure which comprises a Faraday rotator and a crystal rotator. The crystal rotator is an anisotropic medium which restricts the type of polarization of the waves which may be supported in the ring laser to substantially circular polarization and also provides a different optical pathlength for right-hand sense circularly polarized waves than for left-hand sense circularly polarized waves. The Faraday rotator is a nonreciprocal device and provides different time delays to waves of each polarization sense passing in the laser ring in one direction from those of such polarization sense passing in the opposite direction. The combination of the crystal rotator and Faraday rotator provides the four frequency relationship discussed above.
While the described polarization dispersive structure has been found satisfactory in many applications, the use of a crystal rotator increases loss and scatter imparted to the propagating waves and introduces linear birefringence to the ring resonator thereby reducing the accuracy of a laser gyroscope using such polarization dispersive structure.